Out of those philosophy majors that get jobs... Clearly there is another variable influencing the results here, probably a higher tendency for philosophy majors to get graduate degrees.
I'm a bit biased, but in my experience philosophy majors tend to be pretty smart and philosophy degrees don't lend themselves to any particular career path. So I don't find it surprising that there's a big gap between where philosophy majors start and where they end up. And the initial number is probably especially skewed downward by lack of initial direction (I would have been ecstatic if my first job out of college paid $39k).
International Relations (noted in the article) also sounds like a degree that would be popular for pre-law.
My guess.
Skimming the required courses for the prelaw program my college offers, the requirements for that are much more aligned with what's needed for a polsci degree then an IR degree. Theres only two classes from prelaw that overlaps with the IR requirements and one of em's not even being offered anymore. Also, the polsci half of the department offers scholarships aimed at prelaw students specifically. (Technically Political Science and International Relations are the same department here, though they have pretty different requirements and different people in charge of the programs)
To generalize the courses, polsci is more "American political science meets history meets business with a dash of criminal justice" while IR is more "Global political science meets history with dash of anthro".
From the WSJ:
But a year-long survey of 1.2 million people with only a bachelor's degree by PayScale Inc. shows that graduates in these subjects earned 103.5% and 97.8% more, respectively, about 10 years post-commencement.